The present disclosure relates to semiconductor light emitting devices, and specifically to a super luminescent diode (SLD) device capable of emitting blue-violet to red light in the visible range.
Semiconductor light emitting devices such as light emitting diodes (LEDs), laser diodes (LDs), and the like have excellent features such as a small size and a high output, and thus are used in diverse technical fields including IT technologies, such as communication and optical disks, as well as medicine, illumination, etc. In recent years, the use of LEDs as light sources of liquid crystal display devices, such as flat-screen television sets, using liquid crystal panels has been rapidly increasing. Such a liquid crystal display device includes a liquid crystal panel as a transmissive light modulator element, and a light source device disposed on a back surface of the liquid crystal panel emits light to illuminate the liquid crystal panel. The liquid crystal panel controls the transmittance of the light emitted from the light source device, thereby forming an image.
Conventionally, a cold cathode fluorescent lamp (CCFL) has been used as a light source of the light source device. However, in recent years, with the trend toward energy conservation, LED backlight sources using LED chips are being developed. Existing LED backlight sources generally use a mode in which a white LED obtained by mixing a blue LED with a yellow fluorescent material, and are broadly divided into a direct-lit type and an edge-lit type based on arrangement of LEDs. In the direct-lit type, LED light sources are arranged directly under a liquid crystal panel in a grid pattern, and thus the direct-lit type is suitable to a technique to increase the contrast ratio of an image by local dimming in which the brightness of the light sources are controlled region-by-region. However, the direct-lit type has problems, for example, where reducing the thickness is difficult. The edge-lit type includes LED light sources arranged in the periphery of a liquid crystal panel, and the entire panel is illuminated by using a light guide plate, so that reducing the thickness of the panel is easy, and the edge-lit type has the advantage of increasing designability. Also in terms of cost, the edge-lit type has the advantage of reducing the number of mounted LEDs can be reduced.
For edge-lit type backlight sources, characteristics such as high directivity, high polarization performance, etc. are primarily beneficial, but existing LED light sources do not have such characteristics, and are not optimized as light sources. Examples of small-sized light sources having high directivity and high polarization performance include LDs, but the LDs have high coherence of light, and thus have a problem where speckle noise is easily generated.
Thus, as light sources having high directivity, high polarization performance, and low coherence, the inventors of the present application focused attention on super luminescent diodes (SLDs). Like LDs, the SLDs are semiconductor light emitting devices including optical waveguides. In the SLDs, while light spontaneously emitted due to recombination of injected carriers advances toward a light-emitting facet, the light is amplified by receiving a higher gain by stimulated emission, and is emitted from the light-emitting facet. The SLDs are different from the LDs in that formation of optical resonators due to facet reflection is reduced, so that laser oscillation due to the Fabry-Perot mode is not caused. Thus, like ordinary light emitting diodes, the SLDs exhibit an incoherent and wideband spectral line shape, and can generate an output of up to about several tens of milliwatts. In particular, SLDs using nitride semiconductors are expected to serve as high output incoherent light sources which covers from an ultraviolet range to a green color in the visible range.
As described above, when the SLDs, which are light sources having high directivity, high polarization performance, and low coherence, are used as edge-lid backlight sources, the optical coupling efficiency between the SLDs and the light guide plate is improved, and the polarizing plate can be omitted. Thus, the SLDs are expected to serve as higher-performance low-cost backlight sources.
Conventional SLDs will be described with reference to FIGS. 13-14.